Moon phase — elongation from the Sun

0 is the new moon, 0.25 the first quarter, 0.5 the full moon, and 0.75 the last quarter. Those targets come from the elongation, not from which zodiac sign the Moon occupies. The Moon can be full in any sign. A sign page that listed “full moon in Aries” as a permanent fact would be wrong within a month.

The public sign pages do not render the phase number, because the phase is a Sun–Moon relationship and the sign page is a 30° window. The birth chart returns the Moon’s longitude and sign, which is the input you would combine with the Sun’s longitude if you wanted the phase for a birthday.

Illumination is a separate field from the 0–1 phase. Two timestamps can share a similar illumination on either side of the full moon and still be different phases, waxing versus waning. Read both fields when the endpoint is used.

A phase of 0.50 with illumination near 100 is the full moon, and the Moon’s sign that night is whatever 30° bin contains the Moon’s longitude. One month that bin is Aries. Another month it is Libra, opposite the Sun, which is the geometry of a full moon and not a property of Libra. A phase of 0.02 is a young crescent even if illumination is still a few percent. Quote both numbers. Sign pages leave them off so a sector is not stuck with last month’s phase.